1. Field of the Invention
The present invention generally relates to methods and process sequences employed in the manufacture of components. More particularly, this invention relates to a manufacturing method which entails selecting optimal processing conditions by which residual stresses are created in a component at sufficient levels to offset service stresses imposed on the component, such that optimal service life is achieved for the component.
2. Description of the Prior Art
A major inefficiency in the manufacturing of hardened, high precision components is the number of processes or steps necessary to finish the component. As an example, the current method of producing the surface of a bearing race involves forming, annealing, rough turning, hardening, several types of grinding and, finally, abrasive-based superfinishing, as illustrated by FIG. 1. An additional setup is required for each process, with occasional inspections being necessary between processes. Therefore, there is a considerable economic motivation to extend one process or machine tool's capability so that other processes in the production sequence can be eliminated, thus reducing setup times and the complexity of the manufacturing schedule, and gaining significant benefits for flexibility and system efficiency.
The difficulty with which the above benefits are achieved is complicated by the service requirements of a component. Components such as bearing races are often subject to severe in-service stresses that may cause premature failure of the component. A major factor determining the service life of a component is surface integrity, which the industry defines as the result of alterations produced in a surface layer during manufacture of the component and which affect the material properties and performance of the component's surface in service. Typically, three factors are considered for surface integrity: surface finish, microstructure and residual stress. The prior art has long given considerable attention to a component's surface finish and physical and material properties in order to meet design requirements such as strength, fatigue and wear, with finish grinding, honing, lapping, polishing, electropolishing and abrasive superfinishing techniques used to readily achieve surface finishes of 16 microinches Ra and less for components that require a polished finish or superfinish. However, because the shape and orientation of the abrasive particles used in finish grinding and other finishing techniques cannot be controlled, material removal by such techniques likewise cannot be controlled in a manner that will predictably and controllably alter certain surface integrity characteristics, such as residual stresses in the surface. Consequently, the prior art has conventionally relied on additional processing steps, such as shot peening, to improve residual stresses in a component.
Research directed specifically to surface integrity has generated more interest of late, particularly with respect to the effect that certain fabrication practices may have on the microstructure and residual stresses of a component. However, improvements in one surface integrity characteristic have generally been achieved at the expense of others. The prior art has been successful in producing machined surfaces characterized by enhanced surface integrity in terms of minimal effects on microstructure and identifying certain machining parameters that affect residual stresses. However, such efforts have been limited to minimum surface finishes of 32 microinches (0.81 micrometer) Ra, which is inadequate for many applications such as bearing races. Furthermore, the prior art has not succeeded in identifying and selecting processing variables that could specifically enhance the service life of a component based on its particular service environment.
Thus, it would be desirable to provide a method for improving the service life of a component through optimizing fabrication parameters based on intended service loads, including improvements in surface integrity and finish of a component. Ideally, such a method would also be achieved while reducing the number of processing steps necessary to finish the component, and thus reduce setup times and processing complexity.